A HVDC (High Voltage Direct Current) system supplies power by converting AC power from a power plant into DC power, transmitting the DC power, and then inverting the DC power into AC power at a power receiving point. The HVDC system has a loss of power less than an AC transmission type, sot it has high power transmission efficiency. Further, the system can improve stability through line separation and has small inductive disturbance, so it is advantageous in long-distance power transmission.
The HVDC system is installed in a structure called a converter module composed of a plurality of submodules stacked 10 meters high in a plurality of layers. Accordingly, two ways are largely used to check the states of the submodules. One of the ways is to use a monitoring computer in a control room at a remote place. The other one is to stop the system and manually check the states of the modules at the site.
However, since the submodules are installed in a high structure, a worker has to use tools such as an aerial ladder, so it is difficult to check the states and maintain the control board.